Add script to generate changelog

[ayberk]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
Adds a script to generate changelog between two tags. Example output between v1.19 tag and HEAD:

➜  cloud-provider-aws git:(changelog_script) ✗ 02/19 14:14 python3 hack/changelog.py --token $TOKEN --print-only --section-title Release --range v1.19.0-alpha.1.. 
Release
* Replace book homepage with README ([#176](https://github.com/kubernetes/cloud-provider-aws/pull/176), @ayberk)
* update klog library from 2.4.0 to 2.5.0 ([#170](https://github.com/kubernetes/cloud-provider-aws/pull/170), @dineshkumar181094)
* Add documentation for cred provider ([#174](https://github.com/kubernetes/cloud-provider-aws/pull/174), @ayberk)
* Add released versions to README ([#175](https://github.com/kubernetes/cloud-provider-aws/pull/175), @nckturner)
* feat: Helm chart for aws cloud controller manager ([#173](https://github.com/kubernetes/cloud-provider-aws/pull/173), @JESWINKNINAN)
* Merge legacy provider ([#160](https://github.com/kubernetes/cloud-provider-aws/pull/160), @nckturner)
* Add wongma7 to owners ([#172](https://github.com/kubernetes/cloud-provider-aws/pull/172), @nckturner)
* tags: initial implementation of tags ([#149](https://github.com/kubernetes/cloud-provider-aws/pull/149), @nicolehanjing)
* Migrate to mkdocs ([#167](https://github.com/kubernetes/cloud-provider-aws/pull/167), @ayberk)
* Update Documentation ([#165](https://github.com/kubernetes/cloud-provider-aws/pull/165), @ayberk)
* Add ECR creds provider ([#157](https://github.com/kubernetes/cloud-provider-aws/pull/157), @ayberk)
* Pass in cloud config file to initialize cloud provider ([#164](https://github.com/kubernetes/cloud-provider-aws/pull/164), @nicolehanjing)
* Bump k8s.io/kubernetes@v1.20.0 ([#151](https://github.com/kubernetes/cloud-provider-aws/pull/151), @nicolehanjing)
* Fix cloudbuild image name ([#163](https://github.com/kubernetes/cloud-provider-aws/pull/163), @nckturner)
* Fix cloudbuild and simplify Makefile & cloudbuild ([#162](https://github.com/kubernetes/cloud-provider-aws/pull/162), @nckturner)
* Add docs publishing script and improve documentation ([#161](https://github.com/kubernetes/cloud-provider-aws/pull/161), @nckturner)
* Fix build command ([#159](https://github.com/kubernetes/cloud-provider-aws/pull/159), @ayberk)
* Makefile target to build and push image for release ([#138](https://github.com/kubernetes/cloud-provider-aws/pull/138), @nckturner)
* add verify-codegen in CI check ([#153](https://github.com/kubernetes/cloud-provider-aws/pull/153), @nicolehanjing)
* Add cloud config for tags ([#152](https://github.com/kubernetes/cloud-provider-aws/pull/152), @nicolehanjing)
* Bump the go version to v1.15 latest ([#150](https://github.com/kubernetes/cloud-provider-aws/pull/150), @nicolehanjing)
* Update go modules to include latest k8s.io/kubernetes module on v-1.19 ([#146](https://github.com/kubernetes/cloud-provider-aws/pull/146), @nicolehanjing)
* Update Flags section of README ([#147](https://github.com/kubernetes/cloud-provider-aws/pull/147), @ayberk)
* instances: initial implementation of instancesV2 interface ([#131](https://github.com/kubernetes/cloud-provider-aws/pull/131), @nicolehanjing)
* latest manifest should point to v1.19.0-alpha.1, not v1.19.1-alpha.1 ([#140](https://github.com/kubernetes/cloud-provider-aws/pull/140), @andrewsykim)

Special notes for your reviewer:
Copy pasted from the authenticator as is.

Does this PR introduce a user-facing change?:

NONE

[nckturner]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ayberk, nckturner

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